Theme Documentation (#3415)

* adding self CREDITS.md

* add "theme" to link for easier navigation

* add documentation about themes and the files that go in them

* add admin documentation for themes

* fix markdown styling

* fix CSSJanus usage

4 files changed, 50 insertions, 7 deletions

diff --git a/docs/en/admins/01_Index.md b/docs/en/admins/01_Index.md
index 49cd247d5..6798dc768 100644
--- a/docs/en/admins/01_Index.md
+++ b/docs/en/admins/01_Index.md
@@ -9,6 +9,7 @@ Learn how to install, update, and backup FreshRSS, as well as how to use the com
 ## Tutorials and Examples
 
 * [Backing up FreshRSS](05_Backup.md)
+* [Installing themes](11_Themes.md)
 * [Installation on Debian 9/Ubuntu 16.04](06_LinuxInstall.md)
 * [Updating on Debian 9/Ubuntu 16.04](07_LinuxUpdate.md)
 * [Setting Up Automatic Feed Updating](08_FeedUpdates.md)
diff --git a/docs/en/admins/11_Themes.md b/docs/en/admins/11_Themes.md
new file mode 100644
index 000000000..cac3c5890
--- /dev/null
+++ b/docs/en/admins/11_Themes.md
@@ -0,0 +1,17 @@
+# Theming
+
+**Note: Currently personal themes are not officially supported and may be overwritten when updating. Be sure to keep backups!**
+
+**For small theme changes, the official [CustomCSS extension](https://github.com/FreshRSS/Extensions) is recommended.**
+
+Themes should be installed at `FreshRSS/p/themes/theme-name`. Docker users can use 
+
+```
+  -v /home/you/my-theme/:/var/www/FreshRSS/p/themes/my-theme/
+``` 
+
+or a similar method to add their theme to their FreshRSS instance. 
+
+# Creating themes
+
+Information on creating themes can be found in [the developer documentation.](/docs/en/developers/04_Frontend/02_Design)
diff --git a/docs/en/developers/01_Index.md b/docs/en/developers/01_Index.md
index fcd9204af..56a807a6f 100644
--- a/docs/en/developers/01_Index.md
+++ b/docs/en/developers/01_Index.md
@@ -23,7 +23,7 @@ Start by creating your development environment. A guide to setting up FreshRSS's
 ## Frontend Development
 
 * [View files](04_Frontend/01_View_files.md)
-* [Design](04_Frontend/02_Design.md)
+* [Design (Themes/Theming)](04_Frontend/02_Design.md)
 
 ## Minz
-Minz is the homemade PHP framework used by FreshRSS. More information can be found [here](Minz/index.md).
\ No newline at end of file
+Minz is the homemade PHP framework used by FreshRSS. More information can be found [here](Minz/index.md).
diff --git a/docs/en/developers/04_Frontend/02_Design.md b/docs/en/developers/04_Frontend/02_Design.md
index c2e622a08..625fede28 100644
--- a/docs/en/developers/04_Frontend/02_Design.md
+++ b/docs/en/developers/04_Frontend/02_Design.md
@@ -1,11 +1,36 @@
-# Template file
+# Writing a new theme
 
-**TODO**
+**Note: Currently personal themes are not officially supported and may be overwritten when updating. Be sure to keep backups!**
 
-# Writing a new theme
+**As of writing (02-02-2021), support for themes as extensions is under development.**
+
+The easiest way to create a theme is by copying and modifying the base theme (or another of the pre-installed themes). The base theme can be found in [/p/themes/base-theme](https://github.com/FreshRSS/FreshRSS/tree/master/p/themes/base-theme). Themes require:
+- a **metadata.json** file to describe the theme.
+- a **loader.gif** file to use as a loading icon (assuming .loading's background has not been overridden)
+- an **icons** folder containing .svg, .ico, and .png files to override existing icons
+- a **thumbs** folder containing a file, **original.png** that will be used as the preview for the theme
+
+"_template.css" is normally added to the metadata file as a fallback for missing aspects. The file is taken from the base theme. If submitting a pull request for a theme, please know that [pull request themes must include this file.](https://github.com/FreshRSS/FreshRSS/pull/2938#issuecomment-624085450)
 
-**TODO**
+## RTL Support
+
+RTL (right-to-left) support for languages such as Hebrew and Arabic is handled through CSSJanus. To generate an RTL CSS file from your standard file, use `make rtl`. Be sure to commit the resulting file (filename.rtl.css).
 
 # Overriding icons
 
-**TODO**
+To replace the default icons, add an "icons" folder to your theme's folder. Use files with the same name as the default icon to override them. 
+
+# Template file
+
+metadata.json
+```
+{
+  "name": "Theme name",
+  "author": "Theme author",
+  "description": "Theme description",
+  "version": 0.1,
+  "files": ["_template.css", "file1.css", "file2.css"]
+}
+```
+
+An example of a css theme file can be found at [/p/themes/base-theme/base.css](https://github.com/FreshRSS/FreshRSS/blob/master/p/themes/base-theme/base.css)